1. Field of the Invention
The invention refers to a method for concentrating and solidifying salt-containing solutions, in particular borate- or sulfate-containing solutions, in a sealed vessel under the action of heat by means of microwaves, the salt solution to be concentrated being fed into the container continuously or in batches, and liquid being evaporated and then preferably fed into a condensate vessel. The invention also refers to an apparatus for concentrating and solidifying salt-containing, for example borate-containing or sulfate-containing, solutions, comprising a heat source, a vessel sealable by means of a cover having at least one inlet conduit for the salt solution to be concentrated and solidified, and a discharge conduit.
2. Description of Related Art
A method and a device for drying contaminated liquids and solutions is known from DE 35 44 270 A1. In this, the liquid is dried by means of microwaves. To achieve high efficiency, it is proposed that the microwave energy be delivered during the drying operation such that the microwaves are phase-shifted in accordance with the particular fill level inside the vessel and with the varying absorption capability of the contents.
The known features yield the advantage that the heat treatment on the one hand, and the handling necessary therefor on the other hand, can be physically separated from one another, so that radioactive exposure for the operating personnel can be ruled out or greatly reduced.
It is also known to use an electrical heating system, surrounding a drum, for in-drum drying of liquid radioactive wastes (DE 40 53 162 A1, DE 32 00 331 C2). In order to improve the efficiency of the drying of liquid by means of microwave energy, DE 31 09 513 C2 provides for microwave energy-absorbing packing elements which are introduced into a tower reactor through which the liquid to be dried flows.
If it is desired to concentrate and solidify sulfate-containing salt solutions with the known methods, it is possible, for example, to introduce 100 kg of dry substance into a rolling hoop drum having a capacity of 200 liters, 55% water of crystallization being present. Since, in addition, such drums can often be only half filled or three-quarters filled, the actual number of volumes to be disposed of is large, and they require a great deal of room for storage.
EP 0 343 673 A1 discloses a method for producing sodium carbonate in which a heating operation in a temperature range between 32 and 35.3.degree. C. is accomplished in order to reduce the water content. The heating operation can be accomplished by means of microwave energy.
According to U.S. Pat. No. 4,179,493, salt solutions are concentrated by means of direct radiation in the form of sunlight.
A method for solidifying liquid radioactive wastes which are converted into gels with the addition of a silicon solution is known from U.S. Pat. No. 4,514,329. According to JP 58-115066 A, radioactive waste product solutions, with the addition of Si and B compounds, are dried by means of microwaves, roasted, and melted to form synthetic materials.
Borate-containing solutions with pH values &gt;5, usually in the alkaline range, occur in a number of chemical processes. In the operation of pressurized water nuclear reactors, a variety of solutions are reduced in volume by means of a conventional evaporator. Concentrates occur which contain approximately 20 wt % salts, principally boron compounds.
These evaporator concentrates are then subjected to a solidification operation, i.e. are solidified in suitable receptacles, by means of external heating means preferably in the form of jacket heating systems, until the salts containing water of crystallization crystallize out. This evaporation takes place in a final storage receptacle.
The known methods, in which borate-containing solutions having toxic organic constituents such as oils, surfactants, and/or radioactive materials are to be solidified, yield solids in the form of salt blocks which often contain all of the water of crystallization. This means that, for example in the case of borax (Na.sub.2 B.sub.4 O.sub.4.10H.sub.2 O), up to 47 wt % water is contained in the final product. In the case of dihydrated disodium tetraborate (Na.sub.2 B.sub.4 O.sub.7.2H.sub.2 O), it means a 15 wt % water content.